Energy storage by means of hydrogen has been gaining increasing importance recently. There are today various techniques for storing hydrogen, it being possible to distinguish between storage in the gaseous, liquid or, in the form of metal hydrides, in the chemically bound state.
Small quantities of hydrogen can be stored simply in compressed gas bottles. In this respect, the following basic principle applies: the higher the pressure in the tank the higher is the storage density. In connection with the introduction of vehicles driven with natural gas, compressed gas tanks of steel have been developed which are, as a rule, officially approved for a filling pressure of up to 250 bar. Usually, containers are nowadays made of metal in order to counteract the embrittlement tendency of hydrogen and in order to achieve the required compressive strength. Austenitic steels are used predominantly. However, pressure tanks made of steel have the disadvantage that they have a high inherent weight and require cumbersome insulation which considerably restricts the reach of mobile systems.
For the use of fuel cells, small tanks with a high energy density are required which are to guarantee a maximum reach in the mobile sector. For this reason, composite tanks (full composite pressure flasks) were developed which can be filled from time to time up to a pressure of 350 bar. The volume-specific storage density ranges 0.5 kWh/1 for steel bottles to as much as 0.8 kWh/1 for full composite bottles.
Moreover, storage of hydrogen in the liquid phase in cryogenic tanks at temperatures down to −250° C. is common.
Tanks with a metal hydride fill are not subject to the high requirements regarding isolation or pressure. The effective pressure level is between 5 and 50 bar and the temperatures which arise for absorption and desorption of hydrogen are between 20° C. and approximately 280° C., depending on the hydride used.
As a result of the requirements regarding the tank material being relatively low in comparison with other types of storage vessels, those with a plastic jacket are used as a rule. The disadvantage of a plastic jacket is the relatively low molecular density as a result of which diffusion of the hydrogen from the tank is possible. By cross-linking the molecular chain of the plastic material and/or using special diffusion barriers, the diffusion of hydrogen from the tank can be reduced.
Such a plastic tank with a diffusion barrier is disclosed in WO 02/088593 A1, for example. Preferably, a metal-plastic composite film is used as diffusion barrier. In practice, it has been found that even such a diffusion barrier cannot fully exclude the diffusion of hydrogen.